There is nothing that you really need to do, they are already milliseconds, since the era and dates of javascript take milliseconds from the era.
http://jsfiddle.net/QzKwE/9/
var dateVal ="/Date(1342709595000)/";
var date = new Date(parseFloat(dateVal.substr(6)));
document.write(
(date.getMonth() + 1) + "/" +
date.getDate() + "/" +
date.getFullYear() + " " +
date.getHours() + ":" +
date.getMinutes() + ":" +
date.getSeconds()
);
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